Communication devices such as wireless devices are also known as e.g. User Equipments (UEs), mobile terminals, wireless terminals and mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communication network, wireless communication network or wireless communications system, sometimes also referred to as a cellular radio system or a cellular network. The communication may be performed, e.g., between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN), and possibly one or more core networks, comprised within the cellular communication network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablet computer with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communication network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g., a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, “node B” or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as, e.g., macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile). In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs, eNBs or even NBs, may be directly connected to other base stations and may be directly connected to one or more core networks.
The 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the base stations.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. High Speed Packet Access (HSPA) is an amalgamation of two mobile telephony protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), defined by 3GPP, that extends and improves the performance of existing 3rd generation mobile telecommunication networks utilizing the WCDMA. Moreover, the 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies, for example into evolved UTRAN (E-UTRAN) used in LTE.
In the context of this disclosure, the expression Downlink (DL) may be used for the transmission path, or send direction, from a base station to a mobile station. The expression Uplink (UL) may be used for the transmission path, or send direction, in the opposite direction, i.e. from a mobile station to a base station.
Cellular radio systems continue to evolve. Much of the evolvement is driven and standardized by the third generation partnership project (3GPP). One important feature is support of multiple Radio Access Technologies (RATs) and handover of wireless devices between different RATs, i.e. inter-RAT handover, involving RANs of different RATs, e.g. handover from GSM EDGE Radio Access Network (GERAN) to Universal Terrestrial Radio Access Network (UTRAN). However, an inter-RAT handover may fail for various reasons and it is of present and future interest to ensure that this will not happen, or at least reduce the risk of such failure.